Methodology for placement based on circuit function and latchup sensitivity

ABSTRACT

A structure, apparatus and method for circuits to minimize sensitivity to latch. The method includes, for example, identifying element density of at least one functional circuit block and element attributes of elements associated with the at least one functional circuit block. An element density function parameterized from the element attributes is formed. The placement of the at least one functional circuit block is modified relative to other functional circuit blocks based on the element density function to substantially eliminate latching effects in a circuit.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates to integrated circuits, and more particularly to amethod and structure for optimizing latchup robustness in integratedcircuits.

2. Background Description

As electronic components are becoming smaller and smaller along with theinternal structures in integrated circuits, it is becoming easier toeither completely destroy or otherwise impair electronic components fromlatchup. Latchup is when a pnpn structure transitions from a lowcurrent/high-voltage state to a high current/low-voltage state through anegative resistance region (i.e., forming an S-Type I-V(current/voltage) characteristic).

Latchup is typically understood as occurring within a pnpn structure, orsilicon controlled rectifier (SCR) structure. Interestingly enough,these pnpn structures can be intentionally designed, or evenunintentionally formed between structures. Hence, latchup conditions canoccur within peripheral circuits or internal circuits, within onecircuit (intracircuit) or between multiple circuits (intercircuit).

Latchup is typically initiated by an equivalent circuit of acrosscoupled pnp and npn transistor. Parasitic npn and pnp transistorsassociated with an inverter, with a base and collector regionscross-coupled, current flows from one device leading to the initiationof the second (“regenerative feedback”). These pnp and npn elements canbe any diffusions or implanted regions of other circuit elements (e.g.,P-channel MOSFETs, N-Channel MOSFETs, resistors, etc.) or actual pnp andnpn bipolar transistors.

In CMOS, the pnpn structure can be formed with a p-diffusion in ann-well, and an n-diffusion in a p-substrate (parasitic pnpn). In thiscase, the well and substrate regions are inherently involved in thelatchup current exchange between regions.

The condition for triggering a latchup is a function of the current gainof the pnp and npn transistors, and the resistance between the emitterand the base regions. This inherently involves the well and substrateregions. The likelihood or sensitivity of a particular pnpn structure tolatchup is a function of spacings (e.g., base width of the npn and basewidth of the pnp), bipolar current gain of the transistors, substratesheet resistance and spacings (substrate resistance), the well sheetresistance and spacings (well resistance), and isolation regions.

Latchup can be initiated from internal or external stimulus. Forexample, latchup is known to occur from single event upsets (SEU).Single event upsets can include terrestrial emissions from nuclearprocesses, and cosmic ray events, as well as events in spaceenvironments. Cosmic ray particles can include proton, and neutron,gamma events, as well as a number of particle types that enter the earthatmosphere. Terrestrial emissions from radioactive events, such as alphaparticles, and other radioactive decay emissions can also lead tolatchup in semiconductors. Also, latchup can occur from voltage orcurrent pulses that occur on the power supply lines, such as V_(DD) andV_(SS). Transient pulses on power rails (e.g., substrate or wells) cantrigger latchup processes, as well.

Latchup can be initiated by negative transient on the V_(DD) which canlead to a forward biasing of all the n-diffusions and nwell structuresand electron injection throughout the semiconductor chip substrate. Thisproduces a “sea of electrons” injected in the chip substrate.Equivalently, a positive transient on the V_(SS) can lead to holeinjection, and forward biasing of the substrate-well junction providinga “sea of holes” event. Latchup can also occur from a stimulus to thewell or substrate external to the region of the thyristor structure fromminority carriers.

In internal circuits and peripheral circuitry, latchup are both aconcern. Latchup can also occur as the result of interaction of the ESD(electro static discharge) device, the I/O off-chip driver and adjacentcircuitry initiated in the substrate from overshoot and undershootphenomenon. These can be generated by CMOS off-chip driver circuitry,receiver networks, and ESD devices.

In CMOS I/O circuitry, undershoot and overshoot can lead to injection inthe substrate. Hence, both a p-channel MOSFET and n-channel MOSFET canlead to substrate injection. Simultaneous switching of circuitry whereovershoot or undershoot injection occurs, leads to injection into thesubstrate which leads to both noise injection and latchup conditions.Supporting elements in these circuits, such as pass transistors,resistor elements, test functions, over voltage dielectric limitingcircuitry, bleed resistors, keeper networks and other elements can bepresent leading to injection into the substrate. ESD elements connectedto the input pad can also lead to latchup. ESD elements that can lead tonoise injection, and latchup include MOSFETs, pnpn SCR ESD structures,p+/n-well diodes, nwell-to-substrate diodes, n+ diffusion diodes, andother ESD circuits. ESD circuits can also contribute to noise injectioninto the substrate and latchup.

In a semiconductor chip environment, there exists a plurality ofdifferent stimulus as well a plurality of circuit functions. Peripheralcircuits comprise, for example, ESD networks, transmitter and receivernetworks, system clocks, phase lock loops, capacitors, decouplingcapacitors and fill shapes. Internal circuits can consist of DRAMmemory, SRAM memory, gate arrays, and logic circuitry. In this complexenvironment, the latchup event can be an interaction of intercircuitinteraction or intracircuit interactions.

Additionally, the interaction of the different circuits can lead to aninitiation of a primary latchup event followed by a secondary latchupevent. Since the circuits in a complex chip are coupled through thesubstrate, well, and power rails, circuit blocks and elements within acircuit block can be interactive.

Moreover, latchup vulnerability is a function of the macroscopic localtemperature in a region where latchup occurs. Certain regions of afunctional chip are hotter than others with a high local temperature.Regions of a chip where the chip is hotter are more likely to initiatelatchup concerns. The reason this is true is that the parasitic bipolargain increases with increased temperature. Additionally, the diffusioncoefficient is a function of temperature leading to longer diffusionlengths in the region of higher temperatures in a semiconductor chip.

For external latchup issues, there is two concerns. First the intrinsiclatchup parasitic structure is more sensitive to the increasedtemperature leading to a higher risk of latchup. Second, the diffusionof minority carriers to the triggerable network is easier as thetemperature increases as a result of the higher diffusion process. Henceit would be an advantage to place these hotter circuits farther fromregions of overshoot and undershoot injection.

Additionally, latchup is a higher risk in regions where the incomingvoltage exceeds the native voltage. Regions of semiconductor chips thatprovide programmable NVRAM power pins which receive voltages that farexceed the native voltage are sources for latchup concern. Mixed voltageoffchip driver circuits, mixed voltage ESD networks, mixed voltagereceiver networks, mixed voltage ESD power clamps all have higher powersupply voltages than the native power supply voltage of the chip.Placement of circuit blocks and circuit functions with a high density ofpnpn devices, or weakly robust networks that are near the high voltagesupply are more vulnerable to trigger latchup in a chip.

SUMMARY OF INVENTION

In a first aspect of the invention, a method comprises identifyingelement density of at least one functional circuit block and elementattributes of elements associated with the at least one functionalcircuit block. An element density function parameterized from theelement attributes is formed. The placement of the at least onefunctional circuit block is modified relative to other functionalcircuit blocks based on the element density function to substantiallyeliminate latching effects in a circuit.

In another aspect of the invention, a method includes identifyingelement attributes within a functional circuit block and forming anelement density function parameterized from the element attributes. Thefunctional circuit block is placed based on the element density functionto substantially eliminate latching effects in a circuit.

In yet another aspect of the invention, a method comprises forming alayout representation in a function block and defining a mathematicalgraph representation of latchup parasitic in the function block. Alatchup density function from the mathematical graph representation isprovided. The function block in a semiconductor chip is then placed tominimize latchup based on the latchup density function.

In another aspect, the method includes identifying injection source andelements associated with the at least one functional circuit block,evaluating global latchup of injector to functional circuit blockdistance and functional circuit block latchup, evaluating form factorsand placement of the functional circuit block and determining whetherthere is a sufficient cost factor savings based on the form factors andplacement of the functional circuit blocks. If there is not sufficientcost savings, then the method further includes modifying placement ofthe functional circuit block relative to other functional circuit blocksbased on an element density function.

In a further aspect of the invention, an apparatus is provided having acircuit schematic representation of a at least one type of transistorand at least one type of structure. A design system defines the circuitschematic representation of the at least one type of transistor and onetype of structure and a design system evaluates a circuit density andattributes of the at least one type of transistor and one type ofstructure. A design system places functional blocks based on latchupdensity based on the evaluation of the circuit density and attributes.

A computer program product is also provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a map of chip function organization;

FIG. 2 is a map of chip function circuit type within each functionalgrouping;

FIG. 3 is an example demonstrating the latchup between adjacentfunctional blocks;

FIG. 4 illustrates the circuit schematic of a latchup structure;

FIG. 5 illustrates an embodiment of a graph model representing astructure of the latchup circuit apparatus highlighting the pnpnstructure;

FIG. 6 illustrates an embodiment of a graph model representing astructure of the latchup circuit highlighting the coupling between a pnpstructure and an npn structure;

FIG. 7 is a mapping of pnp, npn and pnpn density in a functional blockas a function of pnpn density (number) and strength;

FIG. 8 is a flowchart showing analysis of placement based on the pnpndensity relative to injection sources, strength based on the functionalblock; and

FIG. 9 is a flowchart showing re-optimization of the chip functionalblock placement to optimize chip performance and minimize latchupsensitivity according to the invention.

DETAILED DESCRIPTION

In chip design, latchup sensitivity involves the interaction betweendifferent circuit types and functional blocks. Thus, placement ofcertain functional blocks relative to others and a means of defining theplacement provide an advantage in chip design. The invention relates toa method and structure for optimizing latchup robustness in integratedcircuits by providing a design system and methodology that addresses thecircuit type relative to another circuit type where latchup becomes adesign placement criteria for not just local interaction but globalinteractions of one circuit block to another circuit block (as a resultof the interactive nature of certain circuit function and sensitivityrelative to another circuit function). By way of example, the inventionprovides a method, apparatus and structure that predicts and improvesthe latchup tolerance by addressing:

-   -   global as well as local interactions;    -   the placement of certain circuit types and functional blocks        relative to other functional blocks;    -   global and local temperature and the placement of circuit types        based on the power profile, and temperature distribution in a        semiconductor chip;    -   placement of circuit types with voltages above the native        voltage away from circuit function blocks with a high density        and sensitivity of pnpn circuits;    -   the form factor of a circuit block to minimize the propagation        of latchup across functional blocks to improve the latchup        propagation into sensitive functional blocks; and    -   placement of circuit types with voltages above the native        voltage, temperature distribution, circuit function block        density and sensitivity of pnpn circuits, and co-optimize to        improve the latchup in a semiconductor chip.

As such, the invention is capable of predicting latchup within a circuitfunction and between circuit functions in a complex semiconductor chipand thus improving performance by eliminating latchup conditions.

Referring now to the drawings, FIG. 1 is a map of chip functionorganization. Semiconductor chips consist of functional blocks ofcircuit types. In a chip design there are, for example, DRAM arrays 100,SRAM arrays 105, decoupling capacitors 110, ESD input networks 115, gatearray logic regions 120, an I/O 125 and other circuit functions such as,for example, off-chip driver (OCD) banks, receiver banks, ESD powerclamps, analog circuitry, custom logic, voltage islands, wiring bays,fill shapes and other circuit function. Organization of these functionalblocks are decided by wirability and performance.

As should be well understood, each of these functional blocks havedifferent voltage conditions, overshoot and undershoot characteristics,power profiles, and density of p-channel MOSFETs, and n-channel MOSFETs.For example, Table 1 provides a brief exemplary description of theseconditions. Over/ PNP NPN PNPN Ckt V Under Power T PFET NFET DensityDensity Density SRAM Low Low Low High High High High High DRAM Low LowLow High Low High Low Low OCD Hi High High Hot Big Big Low Low Low ESDHi High High Hot Low Low Low Low Low ESD Hi High High Hot Low Big LowLow Low Power Decap Hi Low Low Low Low Low Low Low Low Gate Lo Low HighHot High High High High High Array Wiring Hi None High Low None NoneNone None None

According to data, each type of functional block has a set of criteriathat can be evaluated to determine the voltage, the power, the localtemperature, the PFET density, the NFET density, the PNP parasiticdensity, the npn parasitic density, the pnpn parasitic density, andother parameters related to latchup optimization. In an actual chipdesign, these values can be obtained from a design system. For example,voltage value is important in that the circuit of interest in thefunctional block will be able to evaluate the voltage margin between theapplied voltage and the parasitic circuit holding voltage within thatfunctional block.

Temperature can be obtained from advanced tools that determine thepower. For example, iEDISON, and other simulators can determine chippower and temperature and extract out a temperature field where theplacement of the chip after an initial guess is defined. There are knowntools that extract interconnect and substrate temperature profile in afull chip cross section.

In regions where the chip is hot, minority carriers will diffuse sinceD=μkT/qandL=(Dτ)^(1/2)

-   -   D=−diffusion coefficient;    -   μ=mobility;    -   K=Boltzmann constant;    -   T=temperature;    -   Q=charge;    -   L=diffusion length; and    -   τ=recombination time.

The diffusion of minority carriers extends the risk of latchup across afunctional block. The propagation of latchup across functional blocksare also important to prevent migration of latchup and soft errordisturbs from latchup injection physics from one block to another. Hencethe form factor and its relation of the adjacent functional blocks andform factor of any functional block can be optimized to minimizelatchup.

Now, PFET density can be obtained in a functional block from anextractor in the number of PFET instances in a functional block of achip. Likewise, NFET density of a functional block can also beevaluated. The density of the PFET and NFET are key to the potentiallatchup disturbances that can occur. PFET disturbs can influence thetiming of some circuits from voltage overshoots. NFET disturbs occurfrom substrate undershoots. When the density of circuits that are PFETsare near a source of overshoot, circuit disturbs and forward bias pnpinjection can occur. Hence, it is an advantage to have a design systemwhere the PFET density of a given functional block is not near a sourceof overshoot.

Likewise, when the density of circuits that are NFETs are near a sourceof undershoot, circuit disturbs and forward bias npn injection canoccur. Hence, it is an advantage to have a design system where the NFETdensity of a given functional block is not near a source of undershoot.

In some circuit types, the circuit may contain only PFETs and no NFETs,or vice versa. In that case, no latchup can occur since no parasiticpnpn structures are evident. As a result, circuit functional blocks thatdo not form p-n-p-n pairs are not sensitive to latchup. Hence if anoptimization is chosen to minimize latchup, the information of the pnpnpairs that are present may be important to understanding whether thefunctional block is latchup sensitive.

In the case of a circuit group which has CMOS logic, PFET and NFETcircuits are typically present to form CMOS inverters. In this case, thedensity of PFETs, and the density of NFETs are high. Hence, theidentification of PFET-NFET pairing, and p-n-p and n-p-n couplingforming a pnp-n may be important. These pnpn circuits are then sensitiveto temperature, power, overshoot, undershoot, external sources, andinjection mechanism (e.g., single event upsets such as alpha particles,cosmic rays, muons, neutron, proton, and other events) . Injection fromadjacent circuit functions can trigger these structures. In this casethe knowledge of the pnpn pair density, pnpn “strength” or latchupresiliency, the orientation and form factor in the functional block andspatial density may be of interest to determine the propagation oflatchup within a functional block.

Modifying the placement of the functional block relative to otherfunctional blocks and identification of the form factor, and spatialdensification or layout optimization in the functional block caninfluence the propagation of latchup throughout the chip. In the processof bit disturbs or soft latchup events, and substrate current injection,it is an advantage to provide circuits types that collect the charge aswell as not propagating the charge transport.

Decoupling capacitor elements are used as fill space or to simply addcapacitance to the semiconductor chip. The decoupling capacitor circuituses NFETs. These networks can bring higher than native voltage to thesemiconductor chip dependent on which power supply they are beingconnected to in the circuit. Experimentally, it has been shown thatdecoupling capacitor banks or fill areas do not lead to the propagationof latchup in a semiconductor chip. Hence the placement of using adecoupling capacitor bank near an injecting source is good to avoid thepropagation of latchup or soft latchup (disturbs). Decoupling capacitorswill have a low density of npns, no pnp density, and hence no pnpndensity. A design system and chip optimization CAD system, as disclosedherein, chooses to place these elements about a semiconductor chip nearan injection source, which provides advantages known to one of skill inthe art.

DRAM arrays utilizing a PFET pass transistor will have an n-well regionholding the DRAM cells. The n-well will serve as a source to collectcharge from an injection source. In this type of circuit, there is ahigh pnp density, no npn density value, and hence no pnpn structures.Hence, as a second example, the invention is capable of determining theplacement of DRAM cells which are isolated from the substrate to avoidthe propagation of latchup.

SRAM cells utilize both a PFET and an NFET in close proximity. LikeGate-array or CMOS logic, this has a high pnp density, npn density and avery high pnpn density. As a result, SRAM should be moved away frominjection sources to avoid latchup propagation, bit disturbs and softlatchup events, as can be determined with the invention.

FIG. 2 is a map of chip function circuit type within each functionalgrouping. A floor plan of a chip is shown with the areas labeled for thefunctional blocks. Semiconductor chips consist of peripheral circuits,CMOS logic, decoupling capacitor banks. The floor planning and adjacencyof different functional blocks on a global level as well as a locallevel is key for external latchup events. The functional groupingsrepresented in FIG. 2 include, for example,

-   -   SRAM, PFETs and NFETs 200;    -   Gate array, NFET, PFET 205;    -   I/O PFET NFET 210;    -   DRAM 215;    -   Decap Cap, NFET 220; and    -   ESD diodes 225.

FIG. 3 is an example demonstrating the latchup between adjacentfunctional blocks. It has been shown experimentally that the effect canpropagate from an ESD diode in peripheral cell, serving as the injectorsource, to an I/O bank. Factors that influence the latchup include, forexample,

-   -   Orientation and placement of the injecting source;    -   Distance between the ESD element and adjacent region of high        pnpn density;    -   Form factors of the functional blocks; and    -   Guard ring utilization between the functional blocks.

A method that addresses both global effects for latchup propagation aswell as local effects of the latchup strength is important forevaluation of latchup effects, as is discussed throughout with relationto the invention.

FIG. 4 illustrates the circuit schematic of a latchup structure. WithinC₁ is a pnp bipolar transistor and a resistor R₁. Within C₂ is a npntransistor and resistor R₂.

Latchup structures can be identified by diffusions that exist in thesilicon substrate. For example, by using the invention, a graph modelwould address the following:

-   -   NPN density—Number of npn per unit area;    -   NPN parameter strength (current gain);    -   PNP density—Number of pnp per unit area;    -   PNP parameter strength (pnp current gain);    -   Parameterization of the NPN density as a function of the        strength parameters statistics;    -   Parameterization of the PNP density as a function of the        strength parameters statistics;    -   NPN density; and    -   PNPN parameter strength (Beta product (β_(pnp)×β_(npn))        relationship including substrate and well resistances). (β_(pnp)        is the bipolar current gain of the pnp transistor and β_(np n)        is the bipolar current gain of the npn transistor.)

FIG. 5 illustrates an embodiment of a graph model representing astructure of the latchup circuit apparatus highlighting the pnpnstructure in accordance with the invention. A latchup structure includesa p+ diffusion inside a n-well structure (500), a n-well region (505), ap substrate (510) and a n+ diffusion in a substrate (515). A lateral pnptransistor of FIG. 4 within C₁ is formed between the p+ shape, n-well,and the p substrate, 500, 505 and 510. A lateral npn bipolar transistorof FIG. 4 within C₂ is formed between the n-well, the p− substrate andthe n+ shape 505, 510 and 515. The base width of the pnp is the spacebetween the p+/n-well junction, and the n-well/substrate junction. Thenpn base width is the space between the n-well/p− substrate edge and then+/p− substrate junction edge. The well resistance is associated withthe distance from the n-well contact to the p+ diffusion in the n-well.The substrate resistance is associated with the distance from the p−substrate contact and the n+ diffusion in the p− substrate. This mayalso extend, in embodiments, to diodes, MOSFET p-channel devices, andbipolar elements. It is understood also that the n-type region in thesubstrate can be a diode, a well, a MOSFET n-channel device and bipolarelements.

FIG. 6 illustrates an embodiment of a graph model representing astructure of the latchup circuit apparatus highlighting the pnpnstructure in accordance with the invention. A first representation ofthe circuit can be shown using a “circle” and “line” structure diagramrepresenting a graph model of the pnpn structure. In thisrepresentation, four regions are created, where each region representsthe information contained associated with the physical diffusions andcorresponding shapes. Forming a p-n-p-n graph, it is possible torepresent the schematic diagram of a pnpn structure. In the left handside (LHS) 605, p shape represents the emitter of the pnp transistordiffusion. The LHS n shape represents the base of the pnp transistor.The right hand side (RHS) 605 p region represents the substrate. The RHSn region represents the n-diffusion. The line 610 connecting the circlesrepresents the interaction between the two circles and contains thecorresponding attributes.

First, in this graph, the line represents metallurgical junctionsbetween the physical regions. Additionally parallel circuit elementsexist as well in parallel with the physical junction. Hence in thisgraph representation, a well resistance exists between the LHS p-nregion. Additionally, a substrate resistance exists between the RHS p-nregion. Hence, in the LHS p-n region, the line attributes represents themetallurgical junction, as well as the well resistor element. On the RHSp-n region, the line attribute represents the metallurgical junction aswell as the substrate resistor element.

In this graph theory representation, the circles also contain otherattributes. The attributes can include, for example, power rail voltageand currents, d.c., voltage values, transient voltage or currentexcursions. The attributes in this representation may also containdimensional area, resistances and other parasitic circuit information.The parameters are extracted from simulators and the information isstored and associated with the attributes of the physical region.

From this graphical model, voltage or current stimulus that influencethe potentials or current injections contained within the p-n-p-nregions influence the potentials and can lead to latchup initiation. Anytype of latchup event, whether SEU (single event upsets), externallatchup processes, voltage or current transient phenomenon can influencethe dynamic response of this graph. For example, substrate injectioninfluences the current observed in the LHS n circle 605 and the RHS pcircle 610. From this model, a pnp as well as a npn can be identified.

FIG. 6 may also illustrate an embodiment of a graph model representing astructure of the latchup circuit highlighting the coupling between a pnpstructure and a npn structure. As a representation, the graph model canbe regarded as a first circle, pnp 605 and a second circle npn 610connected by a line 615. In this representation, the line 615 connectingthe pnp and the npn circle regions 605 and 610, respectively, representsthe crosscoupling and regenerative feedback between the pnp and the npn.In this graph representation, the dimensional information, physics andprocess information are contained within the individual circle (605 or610). Additionally, a physical circuit device model of the pnptransistor and the npn transistor may contain all the I-V information ofthe semiconductor device. In this graphical representation, the line orarc 615 represents the linkage between the two physical domain andcoupling. This higher order representation contains all the deviceattributes and identifies the location of one device relative to anotherdevice.

As should be known, two important conditions exist for the latchup tooccur. The pnp and the npn must be forward active. This means that thesum of the forward voltages must be greater than 1.2 volts, for example(although other voltages may apply). For this to occur, a voltage dropmust occur across the well and substrate resistors of at least 0.6 V,for example. The second condition is that regenerative feedback mustoccur. This requirement can be evaluated as a simple algorithm that theproduct of the pnp and the npn current gain is greater than unity, asshould be known to one of ordinary skill in the art. A second algorithmor equation addresses injected current and the well and substrateresistance value. Additionally, the emitter resistances can also beincluded in the regenerative feedback condition. These parameters orattributes will be used for the evaluation of the “strength” of the npndevices or pnp devices or pnpn devices.

FIG. 7 is a mapping of pnp, npn and pnpn density in a functional blockas a function of pnpn density (number) and strength parameter:

-   -   N_(npn) (S)    -   N_(pnp) (S)    -   N_(pnpn) (S)

In this fashion, it is now possible to define a latchup densityfunctional, where the strength parameters will provide both asensitivity parameter to latchup as well as the density in a globalsystem. In this example, using the function blocks of FIG. 1, forexample, the SRAM 105 includes:

-   -   N_(npn) (500)    -   N_(pnp) (500)    -   N_(pnpn) (500)

Other examples are also shown in FIG. 7.

In view of the above discussion, in the methodology of the invention,there exists a density of pnps and npns within a given functional blockof circuits, as well as pnpn. Visually, these can be defined as withinthe “area” of the functional block. There also exists the possibilitythat the pnp or the npn is formed on the perimeter between thefunctional blocks. Additionally, the pnp of one functional block deviceon the perimeter can be intercoupled with the npn of a second functionalblock. Hence, it is possible to define “area” or volumetric densityfunctions for the interior of the functional blocks and a “surface”density function on the perimeter. It is also possible to define theseas inter-versus intra-functional block parasitic elements. Themethodology of the invention can re-optimize the circuit to minimize thesurface density function of pnpns, npns, and pnps by moving functionalblocks to other locations, or space the functional blocks apart toreduce the coupling strength between these elements. The methodology canform a means to space these apart to either eliminate the density ofthese elements, or the F strength factors of these elements.

In the invention, the “area” placement and area density function isaddressed first in the optimization, followed by the surface densityrelative to adjacent blocks. The density of the perimeter of npn, pnpcan be defined within the block, and the density and correspondingstrength factors are to be addressed after the placement of thefunctional blocks and adjacency is established. Spacing between theseblocks can then be re-optimized to weaken the F parameters, or choose anew location for that functional block.

Latchup Condition and Strength Functional

In the invention, Minimum Condition for Regenerative Feedback addressesthe parasitic current gains and the well and substrate resistor elementsin the latchup circuit.

The parasitic current gains and resistors are extracted from knownspacings, and physical parameters associated with the technology.^(β) _(npn)β_(pnp)>(I _(DD) +I _(RW)β_(npn))/(I _(DD) −I _(RW) −I_(RS)(β_(npn) ⁺¹)/(^(β) _(npn)))and(V _(BE))_(pnp)>0.7(V _(BE))_(npn)>0.7whereI _(RW)=(V _(BE))_(pnp) /R _(S)I _(RW)=(V _(BE))_(npn) /R _(W)

-   -   R_(S) is the substrate resistor and R_(W) is the well resistor        on an n-1 substrate wafer and a p-type well. I is current and β        represents β_(pnp)×β_(npn). The parameter I_(DD) is the current        that flows through the circuit in FIG. 4 from the cop node of C₁        to the bottom node of C₂. It should be understood that the        equations sets provided herein are applicable for an n-1 wafer        but can be easily modified by those of skill in the art for a        p-1 substrate wafer and an n-type well type well.

Substrate resistance is typically provided as in units of Ohm-cm whichis designated as ρ. Substrate resistance is designated as R_(s) wherethe value is a function of the distance between the n+ substrate contactand p+ diffusion of the circuit, and divided by a cross sectional area.Rs=β(L/A)

For the well resistor it is a function of the distance between the p+well contact and the n+ diffusion of the circuit, divided by the widthof the contact.Rw _(=ρ sh (Well))(L/W)

To evaluate the magnitude of the latchup sensitivity of the specificcircuit, it is possible to define a metric where:F=β _(npn)β_(pnp)/[(I _(DD) +I _(RW)β_(npn))/(I _(DD) −I _(RW) −I_(RS)(β_(npn) ⁺¹)/(β_(npn)))]

From this term, if F is less than 1, then latchup does not occur. If Fis small (e.g., much less than 1) then it is a weak phenomenon. Usingthe magnitude of F, one can use this as a latchup margin metric. Again,one of skill in the art can modify these relations for the p− substratecase.

If F>1 then latchup will occur. If F>>1 then the latchup sensitivity ofthe specific circuit is very high. This can be used a strength variablefor a given pnpn structure in a given circuit for a fixed p+ to wellspace, n+ to well space, and a given substrate contact and given wellcontact spacing.

Density Functional

A density functional can be created to evaluate the density of pnpnstructures, pnp structures and npn structures. A function can be createdthat identifies the pnp, the npn and counts the number of theseelements.

Counting pnp Elements

To count the number of the pnp elements, such can be determined bycounting the number of design shapes that allows p+ dopant implants toenter the silicon inside the isolation openings for those containedinside the n well. For example, a block mask named BP over ROX(isolation opening) inside an n well shape (NW) will achieve a p+diffusion. In this fashion, the number of pnp elements can bedetermined. The counting of independent BP shapes indicates the numberof pnp elements. For each independent pnp, a pnp strength parameter canbe evaluated similar to the above expression for the pnpn. This will bea function of the current gain of the pnp and the n-well contact.

In a CADENCE™ environment, the number of instances of a given p-cell canbe counted. The pnp can be counted by evaluation of the type of p-cellsand the number of those p-cells. For example, by evaluating the numberof instances of the PFET device p-cell, it is possible to determine thenumber of PFETs. To evaluate the density of these elements, the totalnumber can be divided by the total area of that functional block ofcircuits.

Counting npn Elements

To count the number of the npn elements, the number of (isolationopenings outside n-well) or isolations which are not can determine thenumber of n diffusions in a functional block (excluding the number ofsubstrate contacts). In this fashion, the number of npn elements can bedetermined. The counting of independent ROX shapes indicates the numberof npn elements. For each independent npn, an npn strength parameter canbe evaluated similar to the above expression for the pnpn. This will bea function of the current gain of the npn and the substrate contactresistance.

In a CADENCE environment, the number of instances of a given p-cell canbe counted. The npn can be counted by evaluation of the type of p-cellsand the number of those p-cells. For example, it is possible to evaluatethe number of instances of NFET transistors. That will be the number. Toevaluate the density of these elements, the total number can be dividedby the total area of that functional block of circuits.

Counting the pnpn Density

Knowing the number of pnp devices, it is possible to determine if thereis a corresponding npn in the region of the pnp. Conversely, knowing thenumber of npn elements, it is possible to determine if there is acorresponding pnp in the region of the npn. In the first case, if an ndiffusion ROX opening is adjacent to the pnp device, then a pnpn isformed. Conversely, if a ROX opening p in an NW with BP, a p-diffusionexists. Then a pnpn is formed and the number can be identified. In aCADENCE environment, p-cells with pnp elements which are adjacent top-cells with NPN elements, then a pnpn can be identified.

It should be understood by one of ordinary skill in the art that, in allthe above cases, a filter distance can be applied if the relativespacing between the pnp and a adjacent ROX or an npn and an adjacent pdiffusion is significant, it can be eliminated (e.g., spatial filteringis possible to simplify calculations truncating very weak F factors).The F factor can be evaluated in a simplified case with an initial guessof the currents, or the elimination of the resistor values for a firstcut, followed by a refined analysis.

Injection-to-Circuit Block Placement

From the above circuit blocks, the number of elements pnpn, and npnp canbe evaluated. Additionally, these densities can be a distribution wherethere is a distribution of different elements of strength F from theabove expression (e.g., N_(npn)(F), N_(pnp)(F), N_(pnpn)(F)).

Now, assuming an injector at a vector position R, and a collectingstructure at vector position r_(i), the relative distance between thelocation of the injecting source and the collecting circuit (i) isdesignated as a vector R-r_(i). The relative distance between thelocation of the injecting source and the collecting circuit j isdesignated as a vector R-r_(j). The relative distance between thelocation of a first circuit at (i) and the second circuit (j) isdesignated as a vector r_(i)-r_(j). A spatial function can beestablished which is the relative strength of injection source at R atthe position (i). Calculation of the functional which is the density ofpnpn elements as a function of the strength factor of each pnpn elementscan be evaluated, and a “chip sector” can be evaluated, as discussed.

A Cost Function is then created where:

-   -   if the N_(pnpn)(F>1) of a first chip region is greater than the        N_(pnpn)(F>1) of a second chip region then the first chip region        is moved farther away from the injection region of a chip.    -   if the N_(pnpn)(F<1) of a first chip region is greater than the        N_(pnpn)(F<1) of a second chip region then the first chip region        is moved closer to the injection region of a chip.    -   if the N_(pnpn)(F)=0 for a first chip region, then move closest        to the injection region of a chip.        Refined Analysis

In the case the evaluation requires better definition, the injectionsource magnitude can be added to the functional based on the initialposition. The injection current can be assumed to be of strengthI _(injection) =[I _(inj)(R))/|R−r _(i)|²] exp−{|R−r _(i)|/(Dτ)^(1/2))}

Letting the value of I_(injection)=I_(RW), the functional F can becalculated for each pnpn in a given chip region. In this case, after theN_(pnpn) are identified, the current term can be placed into theanalysis to evaluate N_(pnpn)(F). In this fashion, the initial guess canbe modified with the current term, assuming it is injection into thewell resistor element. In this fashion, the cost function block can bere-evaluated taking into the relative account of the circuit (i) to theinjection source.

Local Re-Optimization of the Local Circuit When F>1.

In the case that the functional block contains elements whose initialstate is F>1 after placement of the functional block, it is possible toperform a second re-optimization of the local elements by changing thespacing of well contact and substrate contacts. In this case, thecircuit is now fixed and the N_(pnpn)(F>1) elements in the block arethen re-optimized such that N pnpn (F>1)→N pnpn (F<1). This isachievable by moving the substrate and well contacts for these specificelements to reduce the F factor. In this case, the number of potentiallatchup sensitive circuits can be reduced, or eliminated.

Secondary Source Evaluation

In the case that there is a high density of pnpns where (F>1), theinteraction between these can lead to a secondary effect. In thisfashion, a secondary source term can be created where the summation ofelements (j) can influence the element (i) where the currentI _(Secondary (i))=Sum(for all j where F>1)[I _(inj)(j)(R))/|r _(j) −r _(i)|²] exp−{|r _(j) −r _(i)|/(Dτ)^(1/2))}

In this fashion, the secondary elements can contribute to the IRW andthe evaluation of F can be re-evaluated to include local secondarysources. As a result, these terms can be also re-optimized based on thesecondary factors as well.

Optimization Based On Chip Temperature

The placement of the chip segments with N_(pnpn)(F) are placed based ona functional solely based on latchup sensitivity and relative locationto a region of concern.

It should be understood that this method can be applied to a region of achip whose temperature is high. If the metric of interest includestemperature, the chip temperature influences the bipolar current gain ofthe parasitic elements. In this fashion, the geometric placement may bechosen based on peak temperature at location R.

According to the methodology of the invention, the F function iscalculated based on the temperature field at the different spatiallocation. In this fashion, chip book placement could be optimized basedon relative location to peak temperature and the functionalN_(pnpn)(F>1@T). Also, temperature can be used to correct the accuracyof the F factors, or it can be the variable to decide placement relativeto a peak temperature.

FIG. 8 is a flow chart showing steps of the invention. The steps of theinvention may be implemented on computer program code in combinationwith the appropriate hardware. This computer program code may be storedon storage media such as a diskette, hard disk, CD-ROM, DVD-ROM or tape,as well as a memory storage device or collection of memory storagedevices such as read-only memory (ROM) or random access memory (RAM).FIG. 8 (and FIG. 9) may equally represent a high level block diagram ofthe system of the invention, implementing the steps thereof.

FIG. 8, more specifically, shows a method for the analysis of placementbased on the pnpn density relative to injection sources, strength basedon the functional block. Using the latchup density factor, the detailsof the propagation process can be simplified for global placement of thefunctional blocks. This avoids addressing all the inter-coupling issues.More specifically, at step 805, the function block is identified. Atstep 810, the circuit function is identified. At function block 815,npn, pnp, pnpn density is evaluated. At function block 820, the latchupsensitivity of the elements (strength parameters) is evaluated. Atfunction block 825, the latchup density functionals Nnpn, N_(pnp),N_(pnpn) are defined. At function block 830, the circuit block densityis defined. The flow of FIG. 8 quantifies the density circuit functionsof circuit densities and defines the latchup density functions by, forexample, taking the shapes and counting and building distributionfunctions. FIG. 9 is a methodology of an embodiment for re-optimizationof the chip functional block placement to optimize chip performance andminimize latchup sensitivity according to the teachings of theinvention. From the latchup density function, the placement of thefunctional blocks can be chosen relative to injection sources. This canbe addressed using the spatial distance from injection virtual levelsthat can be placed on injection sources. The placement of the book typeor functional block can be addressed and integrated into the timingissue.

In function block 905, the chip performance is evaluated. In functionblock 910, the injection source is identified. In function block 915,the functional block identified. In function block 920, the globallatchup of injector-to-block distance (relative positioning) andfunctional block latchup functional is evaluated. In function block 925,the form factors and placement are evaluated. In function block 930, adetermination is made as to whether this placement provides a sufficientcost factor. If yes, then the process ends at “E”. If the placement doesnot provide a sufficient cost factor, then in function block 940, theform factor and placement are changed. Then the process then revertsback to function block 905 to again provide an evaluation based on thechanged placement and form factor.

Since latchup can propagate across functional blocks, the form factorand size of the circuit blocks are used for the development as well asthe latchup density function. Form factors (e.g., decoupling capacitors)can be used to optimize the floor planning. While the invention has beendescribed in terms of embodiments, those skilled in the art willrecognize that the invention can be practiced with modification withinthe spirit and scope of the appended claims. For example, the inventioncan be readily applicable to bulk substrates.

1. A method comprising the steps of: identifying element density of atleast one functional circuit block; identifying element attributes ofelements associated with the at least one functional circuit block;forming an element density function parameterized from the elementattributes; and modifying placement of the at least one functionalcircuit block relative to other functional circuit blocks based on theelement density function to substantially eliminate latching effects ina circuit.
 2. The method of claim 1, wherein the element density is atleast one of PFET density and NFET density.
 3. The method of claim 2,wherein the modifying placement step includes: placing the NFET densityof a given functional circuit block away from a source of undershoot;and placing the PFET density of a given functional circuit block awayfrom a source of overshoot.
 4. The method of claim 1, wherein the stepof identifying element attributes includes identifying at least one ofsensitive to temperature, power, overshoot, undershoot, externalsources, injection mechanisms, element pair density and strength andlatchup resiliency, orientation and form factor in the at least onefunctional circuit block and spatial density.
 5. The method of claim 1,further comprising providing decoupling capacitor elements to fill spaceor add capacitance to eliminate propagation adjacent an injection sourceto avoid propagation of latchup or soft latchup.
 6. The method of claim1, wherein prior to the modifying placement step, the method furthercomprising the steps of determining at least one of: (i) orientation andplacement of an injecting source; (ii) distance between an ESD elementand adjacent region of high pnpn density; (iii) form factors of the atleast one functional circuit block; (iv) guard ring utilization betweenadjacent functional circuit blocks; (v) NPN density—Number of npn perunit area; (vi) NPN parameter strength (current gain); (vii) PNPdensity—Number of pnp per unit area; (viii) PNP parameter strength (pnpcurrent gain); (ix) parameterization of the NPN density as a function ofstrength parameters statistics; (x) parameterization of the PNP densityas a function of the strength parameters statistics; (xi) PNPN density;and (xii)PNPN parameter strength.
 7. The method of claim 1, whereinidentifying element density of the at least one functional circuit blockincludes identifying element density at a perimeter of the at least onefunctional circuit block to determine a surface density function on theperimeter.
 8. The method of claim 1, wherein the modifying placementstep includes spacing the at least one functional circuit block apartfrom the other functional circuit blocks to reduce coupling strengthbetween elements associated therewith.
 9. The method of claim 1, furthercomprising the step of determining magnitude of latchup sensitivity of aspecific circuit by:F=β _(npn)β_(pnp)/[(I _(DD) +I _(RW)β_(npn))/(I _(DD) −I _(RW) −I_(RS)(β_(npn) ⁺¹)/(β_(npn)))] wherein when (i) F is less than 1, thenlatchup does not occur and (ii) F>1 then latchup will occur.
 10. Themethod of claim 1, wherein the element density function includes:counting independent BP shapes indicating a number of pnp elementsthereby determining a number of p diffusions associated with the atleast one functional circuit block; and counting of independent ROXshapes indicating the number of npn elements associated with the atleast one functional circuit block.
 11. The method of claim 10, furthercomprising determining a number of pnpn and npnp elements based on thenumber of npn elements and pnp elements to determine element density ofthe at least one functional circuit block.
 12. The method of claim 9,wherein: a relative distance between a location of an injecting sourceand a collecting circuit at (i) comprising elements associated with theat least one functional circuit block is designated as a vector R r_(i),where an injector is at a vector position R and a collecting structureis at vector position r_(i); a relative distance between the location ofa first circuit at (i) and a second circuit (j) of the at least onefunctional circuit block is designated as a vector r_(i)r_(j); a spatialfunction is established which is a relative strength of injection sourceat R at the position (i); and wherein a function is created where: (i)if an N_(pnpn)(F>1) of a first chip region is greater than theN_(pnpn)(F>1) of a second chip region then the first chip region ismoved farther away from the injection region; (ii) if the N_(pnpn)(F<1)of the first chip region is greater than the N_(pnpn)(F<1) of the secondchip region then the first chip region is moved closer to the injectionregion; and (iii) if the N_(pnpn)(F)=0 for the first chip region, thenthe first chip region is move closest to the injection region.
 13. Themethod of claim 12, wherein an injection current strength is defined as:I _(injection) =[I _(inj)(R))/|R−r _(i)|²] exp−{|R−r _(i)|(Dτ)^(1/2))}where I injection=I_(RW), wherein when the at least one functionalcircuit block contains elements whose initial state is F>1 afterplacement of the functional circuit block, the method further includesthe steps of: performing a re-optimization of local elements by changingspacing of well contact and substrate contacts associated with theelements.
 14. The method of claim 12, wherein placement is chosen basedon peak temperature at location R.
 15. A method comprising the steps of:identifying element attributes within a functional circuit block;forming an element density function parameterized from the elementattributes; and placing the functional circuit block based on theelement density function to substantially eliminate latching effects ina circuit.
 16. A method, as recited in claim 15, wherein the elementdensity is a parasitic transistor structure.
 17. A method, as recited inclaim 15, wherein the element density function includes the elementattributes relevant to latchup.
 18. A method, as recited in claim 15,wherein the placement of the functional circuit block is based on theposition relative to a injection stimulus.
 19. A method, as recited inclaim 15, further comprising identifying further functional circuitblocks, the function circuit block and the further function circuitblocks include at least one of: (i) peripheral circuits; (ii) ESDnetworks; (iii) ESD Power clamps; (iv) decoupling Capacitors; (v) DRAMarray; (vi) SRAM array; (vii) gate array; and (viii) CMOS logic.
 20. Amethod as recited in claim 15, wherein the element density function andthe element attributes evaluation include at least one of: (i) forming agraphical representation of a transistor structure; and (ii) forming agraphical representation of a latchup structure.
 21. A method comprisingthe steps of: forming a layout representation in a function block;defining a mathematical graph representation of latchup parasitic in thefunction block; providing a latchup density function from themathematical graph representation; and placing the function block in asemiconductor chip to minimize latchup based on the latchup densityfunction.
 22. The method as recited in claim 21, wherein the placementof the function block is associated with an injection source.
 23. Themethod as recited in claim 21, wherein a form factor of the functionblock is formed to minimize latchup sensitivity.
 24. A method comprisingthe steps of: identifying injection source and elements associated withthe at least one functional circuit block; evaluating global latchup ofinjector to functional circuit block distance and functional circuitblock latchup; evaluating form factors and placement of the functionalcircuit block; and determining whether there is a sufficient cost factorsavings based on the form factors and placement of the functionalcircuit blocks, if not, modifying placement of the functional circuitblock relative to other functional circuit blocks based on an elementdensity function.
 25. An apparatus comprising: a circuit schematicrepresentation of a at least one type of transistor; a circuit schematicrepresentation of at least one type of structure; a design system whichdefines the circuit schematic representation of the at least one type oftransistor and one type of structure; a design system to evaluate acircuit density and attributes of the at least one type of transistorand one type of structure; and a design system that places functionalblocks based on latchup density functionals based on the evaluation ofthe circuit density and attributes.
 26. The apparatus of claim 25,wherein the at least one type of transistor is an npn and pnp transistorand the one type of structure is at least a pnpn structure.
 27. Acomputer program product comprising: a computer usable medium havingcomputer readable program code embodied in the medium, the computerprogram product having: computer program for identifying element densityof at least one functional circuit block; computer program foridentifying element attributes of elements associated with the at leastone functional circuit block; computer program for forming an elementdensity function parameterized from the element attributes; and computerprogram for modifying placement of the at least one functional circuitblock relative to other functional circuit blocks based on the elementdensity function to substantially eliminate latching effects in acircuit.